.\" -*- nroff -*-
.\" Copyright (c) 2010-2014 Cisco Systems, Inc.  All rights reserved.
.\" Copyright 2006-2008 Sun Microsystems, Inc.
.\" Copyright (c) 1996 Thinking Machines
.\" $COPYRIGHT$
.TH MPI_Buffer_attach 3 "Jun 10, 2020" "4.0.4" "Open MPI"
.SH NAME
\fBMPI_Buffer_attach\fP \- Attaches a user-defined buffer for sending.

.SH SYNTAX
.ft R
.SH C Syntax
.nf
#include <mpi.h>
int MPI_Buffer_attach(void \fI*buf\fP, int\fI size\fP)

.fi
.SH INPUT PARAMETERS
.ft R
.TP 1i
buf
Initial buffer address (choice).
.TP 1i
size
Buffer size, in bytes (integer).

.SH OUTPUT PARAMETER
.ft R
.TP 1i
IERROR
Fortran only: Error status (integer).

.SH DESCRIPTION
.ft R
Provides to MPI a buffer in the user's memory to be used for buffering outgoing messages. The buffer is used only by messages sent in buffered mode. Only one buffer can be attached to a process at a time.

.SH NOTES
.ft R
The size given should be the sum of the sizes of all outstanding Bsends that you intend to have, plus MPI_BSEND_OVERHEAD bytes for each Bsend that you do. For the purposes of calculating size, you should use MPI_Pack_size. In other words, in the code
.sp
.nf
    MPI_Buffer_attach( buf, size );
    MPI_Bsend( \&..., count=20, datatype=type1, \&... );
    \&...
    MPI_Bsend( \&..., count=40, datatype=type2, \&... );
.fi
.sp
the value of size in the MPI_Buffer_attach call should be greater than the value computed by
.sp
.nf
    MPI_Pack_size( 20, type1, comm, &s1 );
    MPI_Pack_size( 40, type2, comm, &s2 );
    size = s1 + s2 + 2 * MPI_BSEND_OVERHEAD;
.fi
.sp
MPI_BSEND_OVERHEAD gives the maximum amount of buffer space that may be used by the Bsend routines. This value is in mpi.h for C and mpif.h for Fortran.

.SH ERRORS
Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument. C++ functions do not return errors. If the default error handler is set to MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism will be used to throw an MPI::Exception object.
.sp
Before the error value is returned, the current MPI error handler is
called. By default, this error handler aborts the MPI job, except for I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error.

.SH SEE ALSO
.ft R
.sp
MPI_Buffer_detach

